the batch.
    *   Support indicator column in boosted trees.
    *   Prevent `tf.gradients()` from backpropagating through integer tensors.
    *   LinearOperator[1D,2D,3D]Circulant added to `tensorflow.linalg`.
    *   Conv3D, Conv3DBackpropInput, Conv3DBackpropFilter now supports
        arbitrary.
    *   Added `tf.train.Checkpoint` for reading/writing object-based
        checkpoints.